Reading a river gauge: stage and flow
A river gauge shows two numbers that look alike and mean different things: a height in feet, and a flow in cubic feet per second, written cfs. Anglers who learn the difference read the river much better. Neither number tells you whether the fishing is good, but both tell you how much water is moving, and that is where every river trip begins.
Stage: the height of the water
The U.S. Geological Survey (USGS) calls the height of the water surface at a spot on a river the stream stage, also called the gage height. It is the height of the water surface, in feet, above an established level where the stage is zero. USGS says that zero level is arbitrary but often close to the streambed. So a stage of 6 feet does not mean the water is 6 feet deep; it means 6 feet above that gauge's own zero. Stage is useful because it is simple to measure and easy to compare over time at one place.
Flow: how much water is moving
Flow, which USGS calls discharge, is the volume of water moving down a river per unit of time, commonly given in cubic feet per second or gallons per day. In general, USGS says, river discharge is computed by multiplying the area of water in a channel cross section by the average velocity of the water in that cross section. A cubic foot is a box of water a foot on each side, so 1,000 cfs means 1,000 of those boxes going past every second. Move the sliders to see the three parts work together.
An interactive drawing goes here in the app.
How a gauge gets from height to flow
USGS explains that a streamgage cannot practically measure flow all the time. Instead it measures the stage continuously, recording it every 15 minutes, and USGS crews measure the flow from time to time at a wide range of stages, using a current meter or an acoustic Doppler current profiler. Plotting those pairs of stage and flow gives a relationship called the stage-discharge relation, or rating curve. Once the curve exists, the continuous record of stage can be converted to a continuous record of flow. Most USGS gauges send their stage data by satellite to USGS computers, where the flow is estimated and the data are reviewed, and the results are available online in near real time.
The curve is site-specific, and it does not stay the same. USGS says stream channels are constantly changing, through erosion or deposition of the bed, seasonal plant growth, debris or ice, and that new measurements are plotted against the existing curve and the rating adjusted. It says it makes discharge measurements at most gauges every 6 to 8 weeks. Try it: slide the stage, then tell the river a flood moved its bed.
An interactive drawing goes here in the app.
Always read the time
A gauge reading is a record of a moment. Stage is recorded every 15 minutes, so a reading should be recent when a gauge is working, and a number that is hours old is a number from the past. That is why TexCast shows the observed time next to every reading and marks a reading stale when it is more than 90 minutes old. That is our rule and not a USGS one.
An interactive drawing goes here in the app.
What the number means for this river
A number of cfs only means something compared with the river it came from. The same flow is a trickle on a big river and a flood on a small creek, and a river that is normal in spring can be unusual in August. Use the readings to compare a river with its own usual flow, and with what it was doing earlier today. Where TexCast has a flow threshold from a source with a date, it shows it with that source. Where we have no source, we show the number and give no verdict.
A check before you go
An interactive drawing goes here in the app.
On the water
- Look at the river first. If the water looks very different from the gauge, trust what you see.
- Check the gauge again before you leave and after a long day on the water. A rising river can change quickly.
- Note the flow when you catch fish. After a few trips you will know which flows work for you on each river.
Easy mistakes
- Reading the stage as the depth of the river. It is a height above the gauge's own zero.
- Comparing cfs between two different rivers as if they meant the same thing.
- Trusting an old reading. Check its time.
- Treating the gauge as a go or no-go verdict. It tells you where the river was, not whether it is right for you today.
In TexCast
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